In general, active-wear shoes are conventionally constructed to have four separate layers: an outsole, a midsole, an upper top layer and, in most shoes, an insole. The outsole or bottom layer is made of a durable material that extends across the lowest surface of the shoe. The midsole or middle layer is situated atop the outsole to provide a cushioning layer to the wearer's foot. Usually, the upper or top layer, formed of leather, synthetics or other materials, is joined to the midsole. Additionally, in most conventional shoes, an insole is further provided for disposal between the midsole and the wearer's foot.
The midsoles of conventional shoe constructions function as shock absorbers to cushion the wearer's foot. The midsole, which usually comprises a plurality of cushioning elements, has the property of deforming over a large area of surface in response to a downward force exerted on the surface. The compression and decompression characteristics of the midsole layer, which are designed and selected to accommodate a particular weight class or cushionability preferences of wearers, are important not only to the comfort of the shoe but also to the functionality and longevity of the shoe.
Consumers of shoes, in particular running shoes and athletic shoes used for a variety of sports, are concerned foremost with (a) the need to select a shoe that is a correct biomechanical fit for the individual and for the particular athletic utility and (b) the need to know when a shoe needs to be replaced, in particular when the shoe loses its ability to provide sufficient cushioning or shock-absorbing effects and/or when the outsole wear inhibits proper function.
In an industry that encompasses an estimated annual market of several billion dollars worldwide, the issues of cushioning, comfort and proper fit of shoes have far-reaching impact on the rate of consumer re-purchase and product satisfaction. Consumers want a specific type of shoe that is best for their particular body-type and biomechanical character, and one which will compensate for any foot, leg or posture problems they may have. Additionally, the consumer needs to know when their shoes need to be replaced with a new pair, which is a question of some significance since the average investment in athletic shoes is not insignificant. Premature retirement of the shoes by 10-15% could cost the consumer public a significant unwarranted expense but, on the other hand, delayed replacement of shoes can cause pain and severe and debilitating injury.
Most authorities, running books and sports advisors place the useful life of a running shoe, for instance, at between 300 and 500 miles (see for example Running Injury Free, Ellis J., and Henderson, J., Rodale Press, 1994). The reasoning for this is simply that it is the midsole of the shoe that provides for most of the shock absorption but it loses its cushioning ability naturally because the midsole material breaks down due to the constant compression and decompression that occurs during running or other athletic activity. Thus, a need still exists for a customized detection system that is responsive to the individual foot motion and to a specific sport or activity and that, in an individualized way, alerts an athlete or general consumer as to the extent of loss of shock-absorbing capacity of the shoe, and the need for shoe replacement.
Additionally, physical wear of the outsole, the part of the shoe that makes contact with the ground or tread surface, is a serious issue impacting the functionality of a shoe. When outsoles wear down, potentially severe biomechanical imbalances can occur and cause injuries. Further, basic traction can be seriously compromised which is a danger to the wearer on any surface. Hence, there exists a continuing need for a device to detect and to signal the wearer as to the extent of outsole wear and the need to replace the shoe.
The present invention describes shoes having a built-in wear indicator device incorporated into the midsole and/or outsole construction that serves as (a) an indicator of outsole wear, (b) an indicator of the ability (or loss of ability) of the shoe to absorb shock, and (c) the need for shoe replacement. This invention finds application in not only athletic shoes but also safety shoes, corrective shoes and ordinary street shoes. Additionally, specialty shoes, such as ski boots, etc., also have a need for the present invention to ascertain a correct fit and to detect and monitor the progressive wear profile, in order to maintain the proper functioning of specific athletic shoe equipment.